Synchronized phasor measurements are rapidly becoming commonly available in power transmission systems. Their availability impacts the performance of several application functions which rely on real-time measurements. In this paper, the impact of synchronized phasor measurements on the state estimation function will be described. The paper will cover issues related to the problem formulation, optimal placement of the synchronized phasor measurements for network observability, bad data detection and parameter error identification. Different types of available phasor measurement units and ways of accounting for these differences when placing these units will also be discussed.